The applicant organization proposes a method of observing nonlinearity in hearing aids and other audio devices based in digital signal processing, and named a differential spectrogram. The technique allows the observer to hear the signal and see its distortion and frequency response in the form of a display that scrolls horizontally across the screen, changing as nonlinearities, appear. The nonlinearities displayed may take the form of compression, clipping, harmonic distortion or intermodulation distortion. The nonlinearities can be seen in the context of the frequency response of the device or in isolation. The nonlinearity data can be recorded and processed for display in various static formats, or observed while changes or adjustments are made to the device under test. The display parameters are to be set to correspond to known characteristics of auditory signal processing to make the depiction of nonlinearity meaningful. The system can be used with speech, noise, music or any other signal. PROPOSED COMMERCIAL APPLICATION No readily available test method allows real-time observation of nonlinearity in hearing aid performance using speech and other real-world signals. As a result, dispensers can observe not only quantitative data but see the exact interaction of signal and device while listening. There are potential applications in music recording and audio testing as well.